Activity & half life Click on mouse or press ‘Enter’ to begin.

1. Activity & half lifeClick on mouse or press ‘Enter’ to begin.

2. Activity The activity of a radioactive substance is equal to how many decays it undergoes in one second. web site: www.ktaggart.com

3. Unit of activity Activity is measured in bequerels (Bq) 1 bequerel = 1 decay per second web site: www.ktaggart.com

4. Half-life (t1/2) The half-life of a radioactive substance is the average time it takes its activity to halve. web site: www.ktaggart.com

5. Examples of half-lives Half lives vary enormously. some examples: Uranium 238 = 4 500 million years Carbon 14 = 5300 years Radon 224 = 60 seconds Helium 5 = 1 x 10-20 seconds web site: www.ktaggart.com

6. Half-life activity decrease Substance X has a half-life of 1 hour. If at 6 am its activity is measured as 6400 Bq At 7 am its activity should be 3200 Bq At 8 am its activity should be 1600 Bq At 9 am its activity should be 800 Bq At 10 am its activity should be 400 Bq At 11 am its activity should be 200 Bq At 12 noon its activity should be 100 Bq At 1 pm its activity should be 50 Bq web site: www.ktaggart.com

7. Half-life graph for substance X web site: www.ktaggart.com

8. half-life Finding half-life from a graph Select an initial high activity. For example 400Bq and draw a horizontal line to the graph’s line. Draw a vertical line down to the time axis and note the time (about 10 seconds in this case) Repeat the above for half the first activity (200Bq gives about 40s) The half-life is the time between these two activities (about 30 seconds) This method should be repeated for other initial activity values in order to obtain an average value for the half-life. web site: www.ktaggart.com

9. Question 1 A radioactive substance produces 800 decays in 20 seconds. Calculate its activity. activity = decays in 1 second activity = 800 ÷ 20 activity = 40 bequerel web site: www.ktaggart.com

10. Question 2 A radioactive substance produces 14 400 decays in 2 hours. Calculate its activity. activity = decays in 1 second activity = 14 400 ÷ 2 hours worth of seconds activity = 14 400 ÷ (2 x 60 x 60) seconds activity = 14 400 ÷ 7200 activity = 2 Bq web site: www.ktaggart.com

11. Question 3 Substance A has a half-life of 3 hours. If at 8 am its activity is 600Bq, what will its activity be at 2 pm? at 8 am activity = 600Bq; 2 pm is 6 hours later this is 2 half-lives later therefore the activity will halve twice that is: 600  300  150 activity = 150Bq at 2 pm web site: www.ktaggart.com

12. Question 4 Substance B has a half-life of 10 minutes. If at 9 am its activity is 1000Bq, when will its activity be 125Bq? at 9 am activity = 1000Bq 1000  500  250  125 therefore the activity has to halve three times therefore it takes 3 half-lives to drop to 125Bq therefore the total time = 3 x 10 minutes = 30 minutes therefore activity = 125Bq at 9:30 am web site: www.ktaggart.com

13. half-life Question 5 Estimate the half-life of the substance whose decay graph is shown opposite. The half-life is approximately 20 seconds web site: www.ktaggart.com